Device including at least one cylinder with a piston-shaped body which is movable therein



Dec. 20, 1966 H. J. VERBEEK 3,292,501

DEVICE INCLUDING AT LEAST ONE CYLINDER WITH A PISTON-SHAPED BODY WHICH IS MOVABLE THEREIN Filed Dec. 4, v1964 2 Sheets-Sheet 1 FIG] I G i 1 j w/f? 8 INVENTOR.

HENDRIK J. VER B EE K FIG.5

3,292,501 ER WITH A PISTON-SHAPED BODY WHICH IS MOVABLE THEREIN Filed Dec. 4, 1964 Dec. 20, 1966 H. J. VERBEEK DEVICE INCLUDING AT LEAST ONE GYLIND 2 Sheets-Sheet 2 FIG.7

INVENTOR.

HENDRIK J.VERBEEK AGENT shape.

United States Patent 3,292,501 DEVICE INCLUDING AT LEAST ONE CYLINDER WITH A PISTON-SHAPED BGDY WHICH IS MOV- ABLE THEREIN I Hendrik Jozef Verbeek, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Dec. 4, 1964, Ser. No. 415,980 Claims priority, application Netherlands, Dec. 24, 1963, 302,577 Claims. (Cl. 92-144) This invention relates to devices including at least one cylinder with a piston-shaped body which is movable therein and which can vary the volume of a working space in the said cylinder. The operating temperatures in the present device are lower than the temperature of the atmosphere, and a gap is present between the pistonshaped body and the cylinder which gap communicates at one side with the working space and is bounded at its other side by a seal provided between the piston-shaped body and the cylinder. Said seal in operation exhibits a temperature higher than that which prevails in the working space.

Known devices of the kind to which the present invention relates are, for example, cold-gas refrigerators, piston-expansion engines, cold compressors, etc.

In such machines the piston, which varies the volume of the cold working space, usually has thin walls and a comparatively great length. This is to isolate the crank case from the working space and also to permit the seal between the piston and the cylinder to be established at the temperature of the atmosphere. The piston and the cylinder exhibit a minimum difference in diameter at the seals. In the said known devices it is common practice to make the diameter of the piston between its end surface and the seal a little less than that at the seal. This is done, because the elongated piston 'is otherwise liable to bindin the cylinder due to inaccuracies in manufacture and tolerances.

The said known devices have, however, the disadvantage that a comparatively large amount of cold disappears through the walls of the gap and through the medium in the gap between the piston and the cylinder to the ambience, which constitutes a loss for these engines. This loss results in a cold leak and hence in a loss of output.

According to the invention it has surprisingly been found that the said disadvantage may be overcome to a considerable extent by forming the wall parts of the piston-shaped body and/or of the cylinder which constitute the boundaries of the gap so that-the gap widens at least over part of its axialdimension towards the seal.

In a device according to the invention the gap widens in a direction opposite to that inknown devices and this surprisingly gives rise to a considerable decrease in the loss of cold.

According to the invention, from a viewpoint of technical manufacture it is advantageous to give the wall parts of the piston-shaped body and/or of the cylinder which parts constitute the boundaries of the gap a conical The gap thus acquires a conical shape in the axialdirection, the maximum width of the gap being at the side of the seal.

In devices of the present kind the gap invariably has a very small width whichis dependent upon a plurality "ice of factors such as the stroke, the diameter and the length of the piston-shaped body, the temperature gradient occurring in the axial direction and the pressure and pressure variations occurring in the working space, number of revolutions, medium, etc.

In another advantageous embodiment of the device according to the invention, the gap has a width between 0.1 mm. and 0.5 mm. at the side remote from the working space.

Another advantageous embodiment of the device according to the invention is characterized in that the wall parts which constitute the gap-bounding surfaces extend so that the width of gap at any point of the gap is proportional to a value which is less than, or equal to, the coefficient of thermal conductivity of the working medium at the temperature corresponding to this area in operation and higher than, or equal to, the square root of this coefficient of thermal conductivity.

The cold loss has been found to be very small with a gap the width of which varies in this manner.

It has been found that, if the position of the piston in the cylinder is not exactly centered, resulting in differing widths of gap over the periphery, a locally much greater loss of cold results. The total loss of cold increases therefore considerably.

To avoid this, a further advantageous embodiment of the device vaccording to the invention is characterized in that the narrow gap exhibits a relative eccentricity of not more than 20% In another advantageous embodiment the device according to the invention is characterized in that an annular centering element is provided in the gap approximately at the area where the narrow gap opens into the working space, which element exhibits apertures regularly divided over its periphery and through which the gap communicates with the working space. Satisfactory centering of the piston in the cylinder is thus obtained in a simple manner so that additional loss of cold resulting from differences in the width of gap is avoided. I

Further the regularly-divided apertures have 'the effect that the working medium is divided over the periphery of the gap very uniformly.

In order that the invention may be readily carried into effect, several embodiments thereof will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIGURES l, 2 and 3 show, not to scale, several pistoncylinder combinations illustrating the shape of the walls of the gap;

FIGURES 4 and 5 are a sectional view and-a plan view respectively, of a cylinder-piston combination having .a centering ring between the piston and the cylinder;

FIGURES 6 and 7 are sectional views of two embodiments of a cold-gas refrigerator in which the gap between the walls of the displacer and the cylinder widened towards the hot space.

Referring more particularly to FIGURE 1, the reference numeral 1 indicates a cylinder in which apiston 2 is movable. The'piston 2 is coupled by means of a connectingrod 3 to a driving mechanism (not shown). The upper side of the piston 2 can vary the volume of a working space 4. This working space may be, for example, the expansion space of a cold-gas refrigerator or a an expansion piston engine. Consequently, low temperatures prevail in the said space in operation.

The piston 2 comprises two portions 5 and 6, the portion 5 constituting the piston proper. Between the portion 5 and the cylinder 1 there is provided a seal 7, which may be formed as a gap seal, but it is alternatively possible to use piston rings, O-rings or a seal in the form of a rolling diaphragm.

The portion 6 of the piston (the piston cap) has a temperature which increases over its axial dimension towards the seal. This implies that the portion 5 of the piston andhence the seal exhibit a temperature higher than that which prevails in the working space.

As may :be seen from FIGURE 1, a gap 8 exists between the portion 6 of the piston and the wall of the cylinder 1. The piston portion 6 is made conical in shape so that the gap 8 becomes wider towards the seal.

For the sake of clarity the width of the gap is shown with much greater dimensions than is the case in practice. The gaps actually have a width at the hot side which is of the order of magnitude from 0.1 mm. to 0.5 mm.

The gaps become narrower towards the cold side so that the width of the gap is very small at the area where the gap opens into the working space.

As previously mentioned, a temperature variation prevails in the axial direction over the gap which rises from the temperature in the working space to a higher level. Now it is known that the coeflicient of heat conduction (A) of the working medium, which may be, for example, hydrogen or helium, depends on temperature. The coeflicient of heat conduction of the working medium is higher at a higher temperature than at a lower tempera ture.

According to the invention it has been found that very low losses of cold occur when the width of the gap varies in proportion with a value which is lower than, or equal to, the coeflicient of thermal expansion and higher than, or equal to, the square root of thecoefficient of thermal expansion.

The shape of the wall of the piston portion 6 is thus chosen so that for two arbitrary points A and B of the gap there applies:

wherein:

widths at the points A and B,

The loss of cold which occurs when using a normal gap having parallel walls or a gap which is wider at its cold side than at its hot side is thus overcome with an extremely simple means.

Although in FIGURE 1 the wall of the portion 6 is formed as part of a conical periphery with a straight generatrix, it is alternatively possible to form the said wall with a curved generatrix.

Further it is possible to give the wall a stepped shape as shown in FIGURE 2.

As appears from FIGURE 3, it is also possible to form the shaped surface in the wall of the cylinder, but thisis' practical only for devices in which the piston 2 performs only a limited stroke.

FIGURES 4 and 5 show the manner in which the piston 2 may be satisfactorily centered in the cylinder 1 by using a centering ring. The centering ring, designated by the reference numeral 10, has apertures 11 which are regulanly divided over its periphery. This affords the advantage that the working medium is divided over the gap very uniformly and this is advantageous as stated hereinbefore.

In fact, if such uniform division does not exist, the loss of cold is found to become considerable at the areas where more mediums flow into and out of the gap.

FIGURE 6 shows for illustrative purposes a cold-gas refrigerator. This refrigerator comprises a cylinder 21 in which a displacer 22 and a piston 23 are adopted to move out of phase. The displacer and the piston are connected through piston rods 24 and 25, respectively, to a driving mechanism (not shown). The piston 23 can vary the volume of a compression space 26.

This space communicates through a cooler 27, a regenerator 28 and a freezer 29 with an expansion space 30. Between the displacer 22 and the cylinder 21 there exists a gap 31 of a shape similar to that of FIGURE 1.

FIGURE 7 shows a slightly differing construction of a cold-gas refrigerator.

This engine comprises a cylinder 41, in which a com-. pression piston 42 is movable, and a cylinder 43 in which a displacer can move. The compression piston and the displacer are connected through piston rods 44 and 45, respectively, to a driving mechanism (not shown) which can movethe displacer and the compression piston with a given phase difference.

The displacer comprises three portions, namely a displacer cap 46, a portion 47 which houses the regenerator, and sealing portion 48. Further. a cooler 49 is housed in the cylinder 43.

The working medium can flow from a compression space 50 through the cooler 49, the regenerator 47 and apertures 51 in the displacer through a gap 52 along the displacer cap to an expansion space 53.

In this engine the portion 47 of the displacer has a conical shape so that the gap between this portion and the cylinder downwardly directed becomes wider.

A refrigerator of the present construction as realized in practice had the following main dimensions and showed the following conditions in operation:

Length of the displacer 36 mms. Diameter of the displacer 12 mms. Stroke of the displacer 12 mms. Pressure ratio in the expansion space :3.

Pressure level 10 atms. Temperature in the expansion space 25 K.

Speed of rotation 1500 rev./min.

The working medium used was helium.

Under these conditions the width of the widening gap was 0.1 mm. at the cold side and 0.25 mm. at the hot side, while the wall of the gap had a purely conical shape. In this engine the loss of cold has been found to be much smaller than in a corresponding engine in which the walls of the gap extended in parallel.

What is claimed is:

1. A device including a cylinder having a piston reciprocating therein to vary the volume of a medium in the working space in said cylinder comprising a seal between said piston and cylinder, and a gap located between said piston and cylinder which communicates at one side with said working space and is bounded at an opposite side'by said seal, said gap being substantially conical inshape with the wider part of said gap being adjacent to said seal. 2. A device as claimed in claim 1 wherein said conical gap at the narrower portion has a width of .1 mm. which increases to .5 mm. at said wider part.

3. A device as claimed in claim 1 wherein. the periph- 5 eral wall of said piston extends in a manner whereby the width of the gap at any point is proportional to a value which is no more than equal to the coefiicient of heat conduction of said medium at the temperature corresponding to this area in operation, and at least equal to the square root of said coefiicient of heat conduction.

4. A device as claimed in claim 1 wherein the narrow portion of said gap has a relative eccentricity of no more than 20%.

5. A device as claimed in claim 1 further providing an annular centering element in the narrow portion of said gap adjacent to said working space, said centering element having apertures regularly divided over its periphery and through which said gap communicates with said working space.

References Cited by the Examiner UNITED STATES PATENTS Shadall 92-169 X Nordberg 230-183 Lundgaard 60 24 X Lundgaard 60--24 Philipp 92176 X Horowitz et a1 92-161 X Roozendaal et al 60 -24 Meijer 62-6 Hogan 62-6 MARTIN P. SCHWADRON, Primary Examiner.

15 I. C. COHEN, Assistant Examiner. 

1. A DEVICE INCLUDING A CYLINDER HAVING A PISTON RECIPROCATING THEREIN TO VARY THE VOLUME OF A MEDIUM IN THE WORKING SPACE IN SAID CYLINDER COMPRISING A SEAL BETWEEN SAID PISTON AND CYLINDER, AND A GAP LOCATED BETWEEN SAID PISTON AND CYLINDER WHICH COMMUNICATES AT ONE SIDE WITH SAID WORKING SPACE IN SAID BOUNDED AT AN OPPOSITE SIDE BY SAID SEAL, SAID GAP BEING SUBSTANTIALLY CONICAL IN SHAPE WITH THE WIDER PART OF SAID GAP BEING ADJACENT TO SAID SEAL. 